C# .NET - parsing "?" - Quantifier {x,y} following nothing

hi,
When a search for '?' in my text box and replace it using the below code

Regex.replace(string,searchtext,+ searchtext +,regexoptions.ignorecase)

am getting the below error.

parsing "?" - Quantifier {x,y} following nothing
.Kindly help me in solving this issue since am a newbie to asp.net c#.

Thanks in advance

Refer : http://stackoverflow.com/questions/3135581/regex-quantifier-x-y-following-nothing-error